chap6-4.txt

function Node(element) {
   this.element = element;
   this.next = null;
   this.previous = null;
}

function LList() {
   this.head = new Node("head");
   this.find = find;
   this.insert = insert;
   this.display = display;
   this.remove = remove;
   this.findLast = findLast;
   this.dispReverse = dispReverse;
}

function dispReverse() {
   var currNode = this.head;
   currNode = this.findLast();
   while (!(currNode.previous == null)) {
      console.log(currNode.element);
      currNode = currNode.previous;
   }
}

function findLast() {
   var currNode = this.head;
   while (!(currNode.next == null)) {
      currNode = currNode.next;
   }
   return currNode;
}

function remove(item) {
   var currNode = this.find(item);
   if (!(currNode.next == null)) {
       currNode.previous.next = currNode.next;
       // if we try to remove last element in the list via this method it will fail as previous of null.
       if (current.next !== null) {
         currNode.next.previous = currNode.previous;
       }
       currNode.next = null;
       currNode.previous = null;
   }
}

// findPrevious is no longer needed
/*function findPrevious(item) {
   var currNode = this.head;
   while (!(currNode.next == null) && 
           (currNode.next.element != item)) {
      currNode = currNode.next;
   }
   return currNode;
}*/

function display() {
   var currNode = this.head;
   while (!(currNode.next == null)) {
      console.log(currNode.next.element);
      currNode = currNode.next;
   }
}

function find(item) {
   var currNode = this.head;
   while (currNode.element != item) {
      currNode = currNode.next;
   }
   return currNode;
}

function insert(newElement, item) {
   var newNode = new Node(newElement);
   var current = this.find(item);
   newNode.next = current.next;
   newNode.previous = current;
   // if there is next element than it should have previous reference to current one or else it would preserve reference to it's previous element in th list
   if (newNode.next) {
     newNode.next.previous = newNode;
   }
   current.next = newNode;
}
   

var cities = new LList();
cities.insert("Conway", "head");
cities.insert("Russellville", "Conway");
cities.insert("Carlisle", "Russellville");
cities.insert("Alma", "Carlisle");
cities.display();
console.log();
cities.remove("Carlisle");
cities.display();
console.log();
cities.dispReverse();